Automatic transmissions of the type addressed by this invention include several fluid operated torque transmitting devices, referred to herein as clutches, which are automatically engaged and disengaged according to a predefined pattern to establish different speed ratios between input and output shafts of the transmission. The input shaft is coupled to an internal combustion engine through a fluid coupling, such as a torque converter, and the output shaft is mechanically connected to drive one or more vehicle wheels.
The various speed ratios of the transmission are typically defined in terms of the ratio Ni/No, where Ni is the input shaft speed and No is the output shaft speed. Speed ratios having a relatively high numerical value provide a relatively low output speed and are generally referred to as lower speed ratios; speed ratios having a relatively low numerical value provide a relatively high output speed and are generally referred to as upper speed ratios. Accordingly, shifts from a given speed ratio to a lower speed ratio are referred to as downshifts, while shifts from a given speed ratio to a higher speed ratio are referred to as upshifts.
In most transmissions, ratio shifting is carried out by selectively directing the fluid pressure output of a pump, referred to as line pressure, to the various clutches of the transmission through the use of one or more shift valves. To upshift from a lower speed ratio to a higher speed ratio, for example, a respective shift valve is activated (electrically or hydraulically) to initiate the supply of fluid pressure to the upper or target speed ratio (on-coming) clutch. Concurrently, the lower speed ratio (off-going) clutch is released, either by exhausting the fluid pressure supplied to it, or through the provision of a one-way device which overruns when the on-coming clutch achieves the required torque capacity.
In most transmissions, a hydraulic accumulator is used to control the supply of fluid pressure to the on-coming clutch. The accumulator absorbs a controlled volume of fluid in parallel with the on-coming clutch, producing a progressive increase in the clutch apply pressure as a piston of the accumulator strokes to compress an internal damper spring.
The average clutch apply pressure during accumulator stroking varies in direct relation to an accumulator bias or trim pressure which opposes the clutch apply pressure, and it is known that the firmness of a shift can be controlled to a desired value over the life of the transmission through adaptive adjustment of such trim pressure. See, for example, the U.S. Pat. No. 4,283,970 to Vukovich, issued Aug. 18, 1981, and U.S. Pat. No. 4,653,350 to Downs et al., issued Mar. 31, 1987, both of which are assigned to the assignee of the present invention. In these controls, the trim pressure varies with the transmission line pressure, and the line pressure is scheduled as a combined function of a base pressure value determined by table look-up and an adaptive pressure value based on a deviation between actual and desired shift times observed during a previous shift of the same type.
Under most conditions, the on-coming clutch engagement is completed during the stroking of the accumulator piston, minimizing the driveline torque disturbance during the shift. However, if the pressure supplied to the accumulator is too low, the initiation of clutch engagement may be delayed to the point where the accumulator piston completely strokes before the clutch fully engages. At the end of the accumulator stroke, the clutch pressure rapidly rises to line pressure, abruptly completing the clutch engagement. This condition is referred to herein as run-through because the shift is completed after the accumulator has "run-through" its stroke.
In addition to creating a harsh bump or clunk at the end of the shift, run-through disrupts the normal relationship between clutch apply pressure and shift time. As a result, the adaptive control may operate to decrease the line pressure even further in subsequent shifting. Various procedures have been proposed for the purpose of detecting the run-through condition, but none have proven to be sufficiently robust due to normal transmission-to-transmission variability.